Conventional multiple branch-line couplers have been known for a long time.
The book by Montgomery (Montgomery Collection MIT, volume 2, pages 897 to 905) describes a coupler of this type with two coupling arms 2 and 4 which connect two parallel main wave guides 6 and 8, each having a width a and depth b (see FIG. 1).
Later, in 1958, Reed calculated "The multiple branch-line wave guide coupler", using Chebyshev polynomials (I.R.E. TRANS M.T.T., (pages 398 to 403) October, 1958, John Reed). One variant of this coupler is also discussed in an article entitled "Synchronous branch-line coupler" by Matthei at pages 819 to 841 of the book "Microwave filters and coupling" published by McGraw Hill.
In both these cases, two rectangular (TE 10 mode) wave guides are coupled together by E plane T-connections.
The coupling arms are perpendicular to the main wave guides.
The length of each of the coupling arms and the spacing between the axes of the arms is close to .lambda.g/4; where .lambda.g is the average length of the guided wave in the frequency band to be transmitted.
The number of coupling arms required in a given frequency band is a function of the value of the coupling required, the acceptable standing wave ratio (SWR) and the required directivity.
These couplers are conventionally constituted by two metal shells assembled in a junction plane which passes through the centres of the large sides of the main wave guides and of the wave guides which form the coupling arms. These two shells are formed from two machined blocks hollowed out from this junction plane to form the main wave guides and the coupling arms. More precisely a half first main wave guide and a half second main wave guide parallel to the first and having the same width b and depth a/2 are initially formed in each of these blocks, and then a plurality of half coupling arms are formed parallel to one another and having the same length .lambda.g/4. The two shells are then assembled so as to place the main half wave guides opposite one another and the half coupling arms opposite one another.
To obtain high electric performance and in particular low passive or linear losses, it is necessary for the junction plane in a TE 10 mode rectangular wave guide to pass through the centres of the large sides of the main wave guides and of the wave guides which form the coupling arms, i.e. through the centre lines of the large surfaces. Indeed, since there is no longitudinal electromagnetic current in the vicinity of these centre lines perfect mechanical and electrical contact between the two blocks can therefore be avoided without disadvantage.
This is why technicians who produce couplers of this type form them from two shells; this realization is all the more desirable as numerous hyperfrequency components must be integrated in the same metal assembly which can then be completely made of two shells after suitable machining.
In the field of hyperfrequencies whose spectrum lies between one GHz and several hundreds of GHz, numerous machining methods can be used, but at 10 GHz and above, the half coupling arms are not easy to produce because of the small dimensions of the main wave guides. The following methods can be used:
electro-erosion to form the coupling arms perpendicularly to the main wave guides. The large thickness of the wall does not make it possible to obtain good reproductibility by this method.
use of a multi-blade plane for cutting out the coupling arms. This method is mechanically accurate but the tooling is expensive. It takes a long time to adjust tooling and to machine properly.
Preferred embodiments of the present invention provide couplers for multiple branch-line wave guides which have small dimensions and a wide frequency band, a low standing wave ratio and high directivity, using a very simple machining method and standard tooling, the machining time being very short.